Nuclear Magnetic Resonance (NMR) Spectrometry is a tool with a myriad of uses and applications. Physical, chemical, structural, and electronic properties can be divined using NMR. Reaction processes can be analyzed in progress and intermediates discovered using NMR. The theory behind NMR is deeply rooted in physics, and in particular magnetism. The same basic principles that make NMR such a useful technique also provides the medical field with one of their most useful tools, namely Magnetic Resonance Imaging (MRI).
How NMR works
The basic action of NMR is the placement of a molecule within a magnetic field. The spectrometer analyzes the magnetism present in atomic nuclei to produce a spectrum detailing the interaction between these magnetic fields. This spectra relates how atoms are situated in relation to each other. Bonding type and atom location can be determined using NMR.
- A nucleus with an odd atomic number has nuclear spin. Since these nuclei have a charge and are in motion the criterion for a magnetic field are present, and thus a magnetic field is generated.
NMR uses magnetic fields ranging from 500-900 megahertz requiring superconductor magnets which must be cooled using liquid helium (-452F). Liquid nitrogen (-321F) must be used to keep the liquid helium cold. There are three basic steps in NMR analysis namely alignment, disturbance, and relaxation.
Alignment
Once a sample is placed within the NMR’s magnetic field the sample molecules line up in a manner that is determined by the molecules magnetic configuration. Inserting the molecule into the magnetic field exerts a torque upon the nuclei. Because the nuclei are spinning, the nuclei begin to wobble.
- This wobble can be manipulated by applying a radio frequency that matches the frequency of the wobble. This results in the nucleus reversing its spin, called magnetic resonance.
Disturbance
A series of radio pulses are then sent through the sample to disrupt the magnetic equilibrium within the atomic nuclei. These pulses cause the nuclei to absorb the energy and radiate it back out. This returned energy contains a specific resonance frequency, which is directly proportional to the strength of the magnetic field that is applied.
- Only nuclei with an odd number of protons or neutrons can be seen using NMR.
Resonance occurs at differing radiation frequencies for each different nucleus. Even the nuclei of the same atom differ based on the other atoms orientation within the molecule. This makes it possible to differentiate the location of similar atoms on differing functional groups or locations.
Relaxation
Relaxation occurs when the nucleus returns to its original state within the applied magnetic field. Once relaxed the nuclei can be probed again, called population relaxation (T1 relaxation). T1 refers to the time required for the nucleus to return to its original equilibrium spin state. When the nuclei switch spin due to the applied magnetic field the nuclei cannot produce a signal until T1 relaxation occurs. This is called transverse relaxation (T2). Nuclei with long T2 times results in sharp peaks in the obtained spectra. Likewise, short T2 times will result in broad peaks.
Proton shielding
The electrons that surround a proton cause an opposing magnetic field to the induced one. These electrons are in effect shielding the proton from the effects of the magnetic field. This phenomenon is called shielding. This means that the electron density around a proton affects the frequency and thus the location of resonance signals. Decreases in electron density, caused by electronegative groups, result in a decrease in shielding (deshielding) and increases the chemical shift.
- Shielding effects are observed not only in the immediate vicinity of a proton, but can be seen in more distant protons as well. This effect does diminish substantially the further you move away from the electronegative group.
Anisotropy, π bonds, and coupling
Electrons that are involved in π bonding systems result in a non-uniform (anisotropic) magnetic field when placed under NMR. This causes the neighboring protons to experience three magnetic fields; namely:
- the applied field
- shielding field
- the field induced by the π bonds
Coupling occurs when the spectra displays groups of peaks closely located to each other. This occurs because the magnetic fields of neighboring protons exert an influence on the proton in question. The number of peaks is directly related to the number of alignments that a proton can experience in a magnetic field. If two different alignments are possible then a doublet (2 closely located peaks) will be observed. As the number of alignments increases, the possibility that two or more of the alignments will be magnetically equivalent also increases. When this occurs they will show up as a single peak in the spectra.
Proton vs. carbon NMR
The advantage proton NMR has over carbon NMR is that carbon 13 is 400 times less sensitive to NMR than is the case for a proton. This is the reason that carbon NMR spectra takes longer to obtain; however, the spectra obtained is usually easier to interpret because peak overlap is less frequent. Even though carbon NMR spectra can be easier to interpret one can not determine how many hydrogen are attached to each carbon. This often necessitates a proton NMR in addition to the carbon NMR.
NMR vs. Crystallography
Crystallography has traditionally been the method of choice when determining a molecule’s structure. It allows for analysis of larger molecules, whereas NMR is limited to molecules under 60 kilo Daltons. The advantage that NMR has over crystallography is that it is performed in solution allowing for studies involving molecular flexibility, intermediate formation, and interactions with other molecules.
References
Hunt, Ian, “Chapter 13: Nuclear Magnetic Resonance (NMR) Spectroscopy”, chem.ucalgary.ca, accessed November 26, 2010.
National Institute of General Medical Sciences, “The Structures of Life - Chapter 3: The World of NMR: Magnets, Radio Waves, and Detective Work”, publications.nigms.nih.gov, accessed November 26, 2010.
WordIQ. “Nuclear magnetic resonance – Definition”, wordiq.com, accessed November 26, 2010.
Thomas Amos - I am a full time professional in the field of chemical research. I hold a Bachelors of Science in Biochemistry, a Bachelors of Arts in ...
Join the Conversation